In the current mobile network, a great many of PS-based (Packet Switch) networks (E-UTRAN, UTRAN-HSPA) are deployed, but their coverage is neither in the whole network nor in the whole area; in some areas signals of the PS network are very poor, and at that time it is necessary to provide a handover from the PS to the traditional CS (Circuit Switch) (such as UTRAN, GERAN) for users, especially when the user is in a session.
FIG. 1 shows a handover mode approved by the standard organization. Firstly, one call is ongoing between UE1 101 and UE2 and this call is on the basis of a PS bearing of an IMS (IP Multimedia Core Network Subsystem), and its session is anchored on an SCC AS (Service Centralization and Continuity Application Server). During the call, the UE1 101 moves to an area with a better CS network coverage; at the time the UE1 101 sends a received signal test report to an MME (Mobility Management Entity) 102 through the original PS network; when the MME 102 receives the test report and knows a handover to be performed from the PS to the CS, it sends a handover request to an MSC (Mobile Switching Centre) 103 which can cover the UE1; the MSC 103 sends the handover request to the IMS network; since the session is anchored on the SCC AS, the SCC AS updates the session with the UE2 after receiving the handover request; after the update of the session is completed, the MSC replies a handover completion response to the MME, and the MME returns the handover completion response to the UE1; and the UE1, after receiving the handover completion response, disconnects the connection with the PS network, and sets a connect with the CS network through the MSC 103. During the whole handover, both the calling user and the called user are not appreciable to the handover.
For an ongoing call, i.e., the calling sends a calling request and the called is in a ringing status and does not respond, and at the time the calling can hear a customized alerting tone customized by the called or a normal ring tone. For such a call, if the handover condition is met, handover can also be performed.
FIG. 2 is a flow chart of handover of an alerting phase session in the related art, including the following steps:
in 201, the UE1 initiates an originating call request to the IMS network through an I-CSCF (Interrogating Call Session Control Function) or an S-CSCF (Serving Call Session Control Function) of a calling network to ask to make a session with the UE2;
in 202, the I-CSCF or the S-CSCF of the calling network, when receiving the originating call request, forwards the originating call request to the SCC AS according to the registration capability of the UE1;
in 203, the SCC AS anchors that originating call request to make preparations for the subsequent handover;
in 204, the SCC AS then returns the originating call request to the I-CSCF or the S-CSCF of the calling network;
in 205, the I-CSCF or the S-CSCF of the calling network routes the originating call request to an S-CSCF of a called network according to the called user number;
in 206, the S-CSCF of the called network routes the originating call request to the called user UE2;
in 207, the UE2, after receiving the originating call request, sends 180 ringing to the calling user UE1 indicating that the called UE2 has received the request, wherein, the response finally reaches the UE1 along the path of the request from back to front through the S-CSCF of the called network, the I-CSCF or S-CSCF of the calling network, the SCC AS, and the I-CSCF or S-CSCF of the calling network;
in 208, at the moment, due to the change of the position, the UE1 receives a test report of the CS network indicating that the CS network has better signals, and it is necessary to hand over the conversation of the PS to the CS network;
in 209, an access stratum network element (such as MME) informs an MSC, and the MSC initiates a handover request to the IMS network or initiates a handover request through an MGCF (Media Gateway Control Function), wherein the request information passes through the I-CSCF or S-CSCF of the calling network;
in 210, the I-CSCF or S-CSCF of the calling network sends the handover request to the SCC AS;
in 211, the SCC AS, after receiving the handover request, finds the ongoing call of the UE1 to update according to the content in the request, and sends an update request to the S-CSCF of the called network through the I-CSCF or S-CSCF of the calling network to mainly update the media type and sending address of the calling UE1;
in 212, the S-CSCF of the called network, after receiving the update request, updates the call which is ongoing with the UE1;
in 213, the UE2, after completing the update, sends a update completion response to the S-CSCF of the called network;
in 214, the S-CSCF of the called network returns the update completion response to the SCC AS through the I-CSCF or S-CSCF of the calling network;
in 215, the SCC AS, after receiving the update completion response, sends a handover completion response to the I-CSCF or S-CSCF of the calling network;
in 216, the I-CSCF or S-CSCF of the calling network sends the handover completion response to the MSC or sends to the MSC through the MGCF.
At the time, the handover is substantially completed. However, in this handover, the situation that there is a customized alerting tone in the call is not considered, especially, the situation that a customized alerting tone service is provided by a forking mode.
FIG. 3 is a simplified flow chart of providing a customized alerting tone service in the related art, including the following steps:
301, a UE1 initiates an originating call request which carries the media resource information of the UE1 and passes through an IMS network element;
302, the IMS network element routes the calling request to a Customized Alerting Tone Application Server (CAT AS) of the called network;
303, the CAT AS routes the calling request to the called UE2 through the IMS network element;
304, the CAT AS sends a response message of the originating call request to the UE1 by using a conversation 2, and the message carries the customized alerting tone media information and passes through the IMS network element;
305, the IMS network element forwards the response message sending by the CAT AS to the calling UE1, so as to make the UE1 and the CAT AS have been negotiated media resources providing the customized alerting tone service;
306, the UE2 sends the response message of the originating call request to the UE1 by using a conversation 1, and the message carries the media resource information for a normal conversation of UE2, passes through the IMS network element, the CAT AS, and the IMS network element, and reaches the UE1, so as to make the UE1 and the UE2 have been negotiated media resources for the normal conversation.
It can be seen from the above originating call flow that, when the UE1 negotiates with the UE2 the conversation media resource information (referring to as normal media resource information, using the conversation 1), the UE1 also negotiates with the CAT AS the customized alerting tone media resource information (using the conversation 2).
If the handover is performed according to the flow shown in FIG. 2, the SCC AS, after receiving the handover request, only updates the media resource information with the UE2, while the customized alerting tone media resource information is no longer updated. For an ongoing originating call, the calling user may have such an experience: when the phone is getting through, the calling hears the customized alerting tone displayed for him by the called, at the time, the position of the calling changes and it is necessary to hand over from the PS network to the CS network, and after the handover is completed, due to no updated customized alerting tone media, the customized alerting tone heard by the calling is suddenly off, while a normal ring does not display also; and at the time, the calling may think that the calling is failed, and may hang up the phone.